Eight patients with cluster headache were studied by evaluating the heart rate variability on the basis of 24-hour ECG monitoring performed during attacks (critic periods) to demonstrate if any imbalance of the autonomic nervous system was present and to determine its temporal correlation, if any, with the attack. The same monitoring allowed us to compare the patients' data during headache-free periods (intercritic periods) with those of normal controls. Our investigation demonstrated a severe sympathovagal imbalance during spontaneous attacks: the parasympathetic drive is at once increased at the onset with a mild reduction of the sympathetic drive, and all these modifications slowly disappear at the end of each attack. Comparison of the average low-frequency and high-frequency values during intercritic periods showed a significant reduction in the low-frequency values in patients, even when the low-frequency/high-frequency ratio was normal. This is highly suggestive of the presence of an autonomic nervous system dysfunction.